1. The Field of the Invention
The invention generally relates to polarization control in VCSELs. More specifically, the invention relates to using gratings for polarization control.
2. Description of the Related Art
Lasers have become useful devices with applications ranging from simple laser pointers that output a laser beam for directing attention, to high-speed modulated lasers useful for transmitting high-speed digital data over long distances. Several different types of lasers exist and find usefulness in applications at the present time. One type of laser is the edge emitter laser which is formed by cleaving a diode from a semiconductor wafer. Cleaving a diode from a semiconductor wafer forms mirrors that form a laser cavity defined by the edges of the laser diode. Edge emitter lasers may be designed to emit a laser beam more strongly from one of the edges than the other edges. However, some laser energy will be emitted at the other edges. Edge emitter lasers are commonly used when high optical power is needed.
A second type of commonly used laser is known as a vertical cavity surface emitting laser (VCSEL). A VCSEL is formed in part by forming a first mirror from Distributed Bragg Reflector (DBR) semiconductor layers. The DBR layers alternate high and low refractive indices so as to create the mirror effect. An active layer is then formed on the first mirror. A second mirror is formed on the active layer using more DBR semiconductor layers. Thus the VCSEL laser cavity is defined by upper and lower mirrors which causes a laser beam to be emitted from the surface of the laser.
One challenge that exists with the VCSELs mentioned above relates to polarization of optical beams. For example, in communication circuits, if polarized light is emitted from a laser device, the light can be routed using various types of beam splitters and polarization selective filters. However, often polarization in a VCSEL will change from batch to batch and depending on the operating conditions under which the VCSEL is operating. For example, a VCSEL may have one polarization at a given bias current and another polarization at a different bias current. In sensor applications, it is often important to emit a constant polarization because part of the sensing operation relates to detecting differences in polarization. Thus, it would be advantageous to effectively pin polarization in VCSELs.